The former planet Pluto was discovered one year previously by Clyde Tombaugh in the search for Planet X. At that time Pluto was thought to be more massive than Mercury, and was not known to have any moons.

PS. Though, to be fair, by some kind of strange sequential logic Edison should've been Doc's doc in 1955 while Einstein is Doc's dog in 1985, which would make Copernicus Doc's doc in 1931. But, hey, to hell with it, we can make it in-sequential